Field of Invention
This invention relates to the transmission of data on a video signal and, more particularly, to the transmission of data on the viewable portion of an analog video signal wherein data symbols may be spread over several fields to enhance transmission reliability, and wherein the amplitude of the data signals is optimized without degrading the video signal.
There have been attempts in the past to superimpose data on a video signal. The most common approach is to insert data during the horizontal and/or vertical blanking interval, such as in the generation of closed captioning signals. Another approach has been to place the data on the visible portion of the video signal. One advantage of the latter approach is that it may be perceivable to a viewer, and hence it degrades the video signal.
An example of the optical-detection approach is disclosed in Broughton U.S. Pat. No. 4,807,031. The basic technique disclosed in this patent is to represent data by raising and lowering the luminance of successive horizontal lines within some designated viewing area. Because the average luminance of the two adjacent lines remains the same, the effect is not perceptible to the eye, but sensing of the net luminance thereof by an appropriate decoder allows the data to be detected. As described in the Broughton the technique is equivalent to superimposing on the video signal a subcarrier frequency of 7.867 kHz, which can be detected by appropriate filtering. Broughton also teach how to determine which fields should have data superimposed on them. For example, according to Broughton, fields that are too white or too black are not appropriate for the insertion of data.
As used herein, the term video signal applies to any representation of the standard NTSC, PAL or SECAM signals in common use for video transmission including the analog form, directly digitized numerical representations, CCIR 601/656 standards-based digital representations, computer representations such as RGB or YUV, or other digital representations that are simply numerically converted from the directly digitized representation of standard video. (Encoding and decoding from any digitized form is contemplated as long as it can be determined how the signal was digitized and that information is not lost after digitization.)
It is an objective of the present invention to provide a data transmission system which is capable of transmitting data at a very high reliability.
A further objective is to provide a data transmission system which can transmit data on a visible portion of a video signal in a manner which insures that the transmission is substantially invisible.
Yet a further objective is to provide a system and method in which video signals are analyzed dynamically and the data transmission is optimized for each video frame to thereby improve the efficiency and quality of the data transmissions using a parameter determined from the analysis.
A more specific objective of the present invention is to provide a highly accurate and reliable data transmission method and means over the visible portion of a video signal by spreading data over several fields.
Yet another objective is to determine a parameter which may be time and/or spatially dependent and which is indicative of the data carrying capacity of the video signal without a degradation of the images thereof.
A further objective is to provide a method and apparatus wherein the video signal is modulated using this parameter to increase the efficiency of the transmission.
Other objectives and advantages of the invention shall become apparent from the following description of the invention.
Briefly, in the subject system and method data is transmitted in the form of groups of data bits defining symbols. Each symbol is associated with or defined by one of a predetermined number of long sequences of xe2x80x9cchipsxe2x80x9d called PN (pseudo-noise) sequences. The PN sequence transmitted for any symbol is divided into a multiplicity of lines of chips. Each line of chips is transmitted together with its inverse, by embedding or superimposed the chips over pairs of lines of the video signal. For example, in one embodiment each symbol representing four data bits (defining 24=16 symbols) may be associated with one of 16 PN sequences of 80 chips each. Next, the image over which each chip sequence is to be superimposed, is analyzed and a parameter is determined for each pixel. The parameter is used to determine an amplitude for the chip. For example, images with either sharp spatial transformations (indicative of edges) or temporary transformations (indicative of movement) can be used to transmit chips with higher amplitudes without degrading the quality of the image.
The PN sequence to be superimposed on the video signal is divided into four lines of 20 chips each. Each line of chips is modulated in accordance with the parameter and is transmitted in its normal form and with its inverse, so that eight lines of twenty chips each are added to or subtracted from respective line scans of the video signal.
Received pairs of lines are operated upon to extract the 20 chips that they represent. This is done by subtracting one line from the other in order to minimize the effect of the video, and by integrating the difference signal for the duration of each chip. Because each chip in the original PN sequence is added to one line and subtracted from the other, when one line is subtracted from the other, not only is the video effect eliminated or at least minimized, but the magnitude of the chip amplitude is doubled. After all chips are processed in this way to derive integrated chip values, the received code is correlated with each of the 16 possible PN sequences for a best match. The symbol that was transmitted is deemed to be that one whose PN sequence has the highest correlation with the received code.
In another embodiment of the invention, instead of 80 chips, each PN sequence consists of a much larger number of chips. For instances, a PN sequences may span over four fields defining two video frames. For a typical video transmission consisting of 244 lines, a PN sequence consists of 244xc3x974xc3x9720 or 19,520 chips. In this embodiment, in the receiver each receiving frame is correlated with a test frame several times by moving the frame sequented downward, one line at a time, in case vertical sync has been lost. This transmission scheme may be used with or without the amplitude modulation previously described.